Not much.
When no app is ranging for iBeacons in the foreground, iOS only does a Low Energy Bluetooth scan to look for iBeacons for a few seconds out of every several minutes. See: http://developer.radiusnetworks.com/2013/11/13/ibeacon-monitoring-in-the-background-and-foreground.html.
I do not know the exact numbers in terms of mAh used, but I would be surprised if it had any significant drain on battery at all. It is much less than the display, GPS, WiFi, Cell Radio, etc.